1. Field of the Invention
The present invention relates to an exhaust gas purifying device for an engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 4-175416 discloses an exhaust gas purifying device for an engine provided with a catalyst arranged in an exhaust passage of the engine, composed of zeolite carrying a transition metal or a precious metal thereon, which is so-called a zeolite catalyst, and a catalyst temperature control device for controlling a temperature of the zeolite catalyst to increase and lower the catalyst temperature alternately and repeatedly in a so-called temperature window of the zeolite catalyst.
On the other hand, the inventors of the present invention have found that the zeolite catalyst has a NO.sub.X adsorbing function in which the catalyst temporarily adsorbs the nitrogen oxides NO.sub.X in the inflowing exhaust gas therein, and that the NO.sub.X adsorbing capacity of the zeolite catalyst becomes lower when the catalyst temperature becomes lower. Thus, lowering the catalyst temperature increases the NO.sub.X adsorbing capacity of the zeolite catalyst, and therefore, the zeolite catalyst temporarily adsorbs NO.sub.X in the inflowing exhaust gas. Contrarily, increasing the catalyst temperature decreases the NO.sub.X adsorbing capacity of the zeolite catalyst, and therefore, the zeolite catalyst desorbs the adsorbed NO.sub.X. Namely, increasing and lowering the catalyst temperature alternately and repeatedly causes the zeolite catalyst to perform the NO.sub.X adsorption and the NO.sub.X desorption and purification, alternately and repeatedly.
Further, the inventors of the present invention have also found that, if the inflowing exhaust gas includes a reducing agent, such as hydrocarbon (HC), almost all of NO.sub.X desorbed from the zeolite catalyst is reduced and purified in an oxidizing atmosphere by the reducing agent. Thus, it is preferable for purifying NO.sub.X sufficiently in the exhaust gas by the zeolite catalyst, to make the NO.sub.X amount adsorbed in the zeolite catalyst as large as possible when the catalyst temperature drops. It is also preferable, for making the adsorbed NO.sub.X amount as large as possible when the catalyst temperature drops, to make the NO.sub.X amount desorbed from the zeolite catalyst as large as possible when the catalyst temperature rises.
As described before, the inventors of the present invention have found that the NO.sub.X adsorbing capacity of the zeolite catalyst becomes larger when the catalyst temperature becomes lower. In other words, the NO.sub.X amount to be adsorbed in the zeolite catalyst becomes larger as the catalyst temperature becomes lower, when the catalyst temperature drops. The NO.sub.X amount to be desorbed from the zeolite catalyst becomes larger as the catalyst temperature becomes higher, when the catalyst temperature rises. Namely, it is most effective for a sufficient NO.sub.X purification to lower the catalyst temperature to a temperature at which the NO.sub.X adsorbing capacity almost becomes a maximum, and increase the catalyst temperature to a temperature at which the NO.sub.X adsorbing capacity almost becomes zero.